1. Field of the Invention
The present invention relates to a method of evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas, such as a catalytic converter for use on an automobile or a hybrid vehicle.
2. Description of the Related Art
Conventional processes of determining the deteriorated state of a catalytic converter for purifying exhaust gases produced when an air-fuel mixture is combusted, e.g., a catalytic converter disposed in the exhaust passage of an internal combustion engine, are known from Japanese patent publication No. 2,526,640 and Japanese laid-open patent publication No. 7-19033, for example.
The disclosed techniques are based on the fact that when the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine is changed from a leaner value to a richer value or from a richer value to a leaner value, the outputs from oxygen concentration sensors that are positioned respectively upstream and downstream of a catalytic converter combined with the internal combustion engine are inverted. More specifically, under certain operating conditions of the internal combustion engine, i.e., when the power output of the internal combustion engine is increased or the fuel supplied to the internal combustion engine is cut off as disclosed in Japanese patent publication No. 2,526,640 or when certain conditions are satisfied, e.g., the load and rotational speed of the internal combustion engine are in predetermined ranges as disclosed in Japanese laid-open patent publication No. 7-19033, the air-fuel ratio is positively changed from a leaner value to a richer value or from a richer value to a leaner value. At this time, the time consumed after the output of the upstream oxygen concentration sensor is inverted until the output of the downstream oxygen concentration sensor is inverted, and the period at which the output of the downstream oxygen concentration sensor is inverted are measured, and the deteriorated state of the catalytic converter is evaluated based on the measured values.
According to these techniques, when the internal combustion engine is operating under ordinary conditions, i.e., conditions without determining the deteriorated state of the catalytic converter, the air-fuel ratio is feedback-controlled depending on the inversion of the outputs from the oxygen concentration sensors in order to keep the air-fuel ratio of the internal combustion engine in the vicinity of a stoichiometric air-fuel ratio, for thereby allowing the catalytic converter to keep an appropriate purifying capability.
However, the above processes of evaluating the deteriorated state of the catalytic converter have suffered the following difficulties:
In order to determine the deteriorated state of the catalytic converter, the air-fuel ratio of the internal combustion engine needs to be positively changed to a leaner value or a richer value. Therefore, while the air-fuel ratio of the internal combustion engine is being feedback-controlled in order to allow the catalytic converter to keep an appropriate purifying capability, it is not possible to determine the deteriorated state of the catalytic converter. When the deteriorated state of the catalytic converter is determined, it is difficult to keep an appropriate purifying capability of the catalytic converter.
According to the conventional processes, the operating state of the internal combustion engine which is capable of determining the deteriorated state of the catalytic converter or the state in which exhaust gases are generated by the internal combustion engine in that operating state is limited to a certain special state. Specifically, according to the process disclosed in Japanese patent publication No. 2,526,640, the deteriorated state of the catalytic converter can be determined only if the output of the downstream O2 sensor is produced in a leaner air-fuel ratio range when the output power of the internal combustion engine is to be increased and at the time of starting to increase the output power of the internal combustion engine, and only if the output of the downstream O2 sensor is produced in a richer air-fuel ratio range when the supply of fuel to the internal combustion engine is to be cut off and at the time of cutting off the supply of fuel to the internal combustion engine.
According to the process disclosed in Japanese laid-open patent publication No. 7-19033, the deteriorated state of the catalytic converter can be determined only if the load (represented by the intake air rate, the throttle valve opening, the fuel injection quantity, and the intake air pressure) and the rotational speed of the internal combustion engine fall in a predetermined range, the intake air temperature is equal to or higher than a preset value, and the load of the internal combustion engine varies by an amount equal to or greater than a preset value. Therefore, if the internal combustion engine which generates exhaust gases to be supplied to the catalytic converter, which may be disposed in the exhaust passage of the internal combustion engine, operates in various operating states or the exhaust gases are generated in various states, then there are not many opportunities to be able to determine the deteriorated state of the catalytic converter, and the reliability of the determined deteriorated state of the catalytic converter under such conditions is low.
The applicant of the present application has proposed a system having a first exhaust gas sensor disposed upstream of a catalytic converter for generating an output representing the air-fuel ratio of an air-fuel mixture combusted by an internal combustion engine, and a second exhaust gas sensor disposed downstream of the catalytic converter for generating an output representing the concentration of a certain component of exhaust gases, e.g., the concentration of oxygen, the system being arranged to control the air-fuel ratio of the internal combustion engine according to a feedback control process to achieve an optimum purifying capability of the catalytic converter based on outputs from the sensors (see Japanese laid-open patent publication No. 9-324681 and U.S. Pat. No. 5,852,930 and Japanese laid-open patent publication No. 11-93740 and U.S. Pat. No. 6,079,205).
The proposed system determines a target air-fuel ratio for the internal combustion engine to cause the output (the detected value of the oxygen concentration) of the second exhaust gas sensor to have a given constant value, and feedback-controls the air-fuel ratio of the internal combustion engine to converge the output (the detected value of the air-fuel ratio) of the first exhaust gas sensor to the target air-fuel ratio, for thereby achieving the optimum purifying capability of the catalytic converter.
Since the system can stably achieve the optimum purifying capability of the catalytic converter according to the above air-fuel ratio control process, it is desirable to be able to evaluate the deteriorated state of the catalytic converter while performing the air-fuel ratio control process.
It is therefore an object of the present invention to provide a method capable of appropriately evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas in various states in which an exhaust gas to be purified by the catalytic converter is generated or in various states in which an internal combustion engine that generates the exhaust gas is operated.
Another object of the present invention is to provide a method capable of appropriately evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas while maintaining a desired purifying capability of the catalytic converter which is disposed in the exhaust passage of an internal combustion engine.
To achieve the above objects, there is provided in accordance with the present invention a method of evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas produced when an air-fuel mixture is combusted, comprising the steps of supplying the exhaust gas downstream to an exhaust passage which has a first exhaust gas sensor and a second exhaust gas sensor that are disposed respectively upstream and downstream of the catalytic converter, for generating respective outputs depending on components of the exhaust gas, detecting data of the outputs of the first exhaust gas sensor and the second exhaust gas sensor when the exhaust gas is supplied to the exhaust passage, sequentially identifying the value of at least one parameter to be set of a model that is constructed as representing a behavior of an object exhaust system which ranges from the first exhaust gas sensor to the second exhaust gas sensor and includes the catalytic converter in the exhaust passage, based on the detected data of the outputs of the first exhaust gas sensor and the second exhaust gas sensor, and determining data representing a degree of variation of time-series data of the identified value of the parameter of the model, as a deterioration evaluating parameter, from the time-series data of the identified value, and evaluating a deteriorated state of the catalytic converter based on the determined deterioration evaluating parameter.
Studies made by the inventors indicate that a model expressing the behavior of the object exhaust system including the catalytic converter and ranging from the first exhaust gas sensor to the second exhaust gas sensor is constructed, and when the value of the parameter to be set, i.e., the parameter to be set to a certain value in defining the behavior of the model, is sequentially identified based on the data of the output of the exhaust gas sensors that are acquired while the exhaust gas is being supplied to the exhaust passage, the time-series data of the identified value of the parameter exhibits a certain characteristic tendency against the deteriorated state of the catalytic converter. Specifically, if the deterioration of the catalytic converter is small, the time-series data of the identified value of the parameter varies to a small extent, and if the deterioration of the catalytic converter becomes larger, the time-series data of the identified value of the parameter varies to a relatively large extent. Therefore, as the deterioration of the catalytic converter progresses, the variation of the identified value of the parameter of the model tends to become larger. This is considered to be due to the fact that the matching between the model of the object exhaust system and the actual behavior of the object exhaust system is lowered as the deterioration of the catalytic converter progresses.
According to the present invention, in evaluating the deterioration of the catalytic converter, the data representing the degree of variation of time-series data of the identified value of the parameter of the model is determined as the deterioration evaluating parameter from the time-series data of the identified value, and the deteriorated state of the catalytic converter is determined based on the determined deterioration evaluating parameter.
Preferably, the first exhaust gas sensor comprises a sensor for producing an output representing the air-fuel ratio of the air-fuel mixture from which the exhaust gas entering the catalytic converter is produced, and the second exhaust gas sensor comprises a sensor for producing an output representing the content of a particular component of the exhaust gas that has passed through the catalytic converter.
When the value of the parameter of the model of the object exhaust system which employs the above sensors as the first and second exhaust gas sensors are identified based on the data of the outputs of the first and second exhaust gas sensors at the time the exhaust gas is supplied to the exhaust passage, the above tendency of the identified value against the deteriorated state of the catalytic converter tends to appear relatively easily. Therefore, it is easy to evaluate the deteriorated state of the catalytic converter based on the deterioration evaluating parameter which represents the degree of variation of time-series data of the identified value of the parameter.
Since it is possible to evaluate the deteriorated state of the catalytic converter when the exhaust gas to be supplied to the catalytic converter is generated in various states or the internal combustion engine operates in various operating states, the catalytic converter is preferably disposed in the exhaust passage of the internal combustion engine which combusts the air-fuel mixture therein.
If the first exhaust gas sensor comprises a sensor for producing an output representing the air-fuel ratio of the air-fuel mixture, and the second exhaust gas sensor comprises a sensor for producing an output representing the content of a particular component of the exhaust gas that has passed through the catalytic converter, then the method preferably further comprising the step of controlling the air-fuel ratio of the internal combustion engine in order to converge the output of the second exhaust gas sensor to a predetermined target value when the exhaust gas is supplied to the exhaust passage upon operation of the internal combustion engine, wherein the value of the parameter is identified and the deteriorated state of the catalytic converter is evaluated concurrent with the step of controlling the air-fuel ratio of the internal combustion engine.
By controlling the air-fuel ratio of the internal combustion engine, or more specifically the air-fuel ratio of the air-fuel mixture combusted in the internal combustion engine, to converge the output of the second exhaust gas sensor which represents the content of the particular component of the exhaust gas having pass through the catalytic converter, it is possible to achieve a desired purifying capability of the catalytic converter for purifying the exhaust gas emitted from the internal combustion engine. When the value of the parameter is identified and the deteriorated state of the catalytic converter is evaluated concurrent with the step of controlling the air-fuel ratio of the internal combustion engine, the deteriorated state of the catalytic converter can be evaluated while maintaining the desired purifying capability of the catalytic converter during operation of the internal combustion engine.
If an oxygen concentration sensor (O2 sensor) is used as the second exhaust gas sensor, then an optimum purifying capability of the catalytic converter is achieved by controlling the air-fuel ratio of the internal combustion engine to keep the output of the sensor at a given constant level.
The step of controlling the air-fuel ratio of the internal combustion engine preferably comprises the steps of calculating a target air-fuel ratio of the internal combustion engine in order to converge the output of the second exhaust gas sensor to the target value, and controlling the air-fuel ratio of the internal combustion engine according to a feedback control process in order to converge the air-fuel ratio represented by the output of the first exhaust gas sensor to the target air-fuel ratio.
By thus controlling the air-fuel ratio of the internal combustion engine, the air-fuel ratio detected by the first exhaust gas sensor can stably be controlled at an air-fuel ratio suitable to achieve the desired purifying capability of the catalytic converter, i.e., the target air-fuel ratio. Since the air-fuel ratio of the internal combustion engine is stably controlled, the behavior of the data of the outputs of the first and second exhaust gas sensors which are used to identify the value of the parameter is made smooth. As a result, the effect of disturbances other than the deteriorated state of the catalytic converter on the identified value of the parameter is reduced. Consequently, the deteriorated state of the catalytic converter can appropriately be evaluated based on the deterioration evaluating parameter which represents the degree of variation of time-series data of the identified value of the parameter.
While the target air-fuel ratio can be calculated using a PID controller, it is preferably calculated by a sliding mode controller.
Specifically, the sliding mode controller is advantageous in that it is more resistant to disturbances than the PID controller. The target air-fuel ratio calculated by the sliding mode controller makes stable the process of controlling the air-fuel ratio. As a result, the desired purifying capability of the catalytic converter can be achieved more reliably. At the same time, because the effect of disturbances other than the deteriorated state of the catalytic converter on the identified value of the parameter is minimized, the deterioration evaluating parameter which represents the degree of variation of time-series data of the identified value of the parameter is made reliable as being highly correlated to the deteriorated state of the catalytic converter. Thus, the deteriorated state of the catalytic converter can be evaluated more adequately based on the deterioration evaluating parameter.
In controlling the air-fuel ratio of the internal combustion engine concurrent with evaluating the deteriorated state of the catalytic converter, the target air-fuel ratio is preferably calculated by an algorithm determined in advance using the identified data of the parameter.
Specifically, since the identified value of the parameter reflects the actual behavioral characteristics of the object exhaust system, when the target air-fuel ratio for converging the output of the second exhaust gas sensor to the target value is calculated using the identified value, the accuracy of the target air-fuel ratio is increased. As a consequence, the desired purifying capability of the catalytic converter can be achieved more reliably.
While the air-fuel ratio of the internal combustion engine can be feedback-controlled by a PID controller, it is preferably controlled by a recursive-type controller.
When the air-fuel ratio of the internal combustion engine is feedback-controlled by a recursive-type controller, or specifically an adaptive controller, it is possible to feedback-control the air-fuel ratio detected by the first exhaust gas sensor more accurately at the target air-fuel ratio while suppressing the effect of characteristic changes of the internal combustion engine than if a PID controller is used. The desired purifying capability of the catalytic converter can be achieved more reliably, and the reliability of the deterioration evaluating parameter which represents the degree of variation of time-series data of the identified value of the parameter is increased, so that the deteriorated state of the catalytic converter can be evaluated more adequately based on the deterioration evaluating parameter.
The recursive-type controller determines a new manipulated variable according to a given recursive formula including time-series data in the past prior to the present of a manipulated variable for the air-fuel ratio of the internal combustion engine, or more specifically a manipulated variable for the fuel supply quantity of the internal combustion engine, for example, in order to converge the air-fuel ratio represented by the output of the first exhaust gas sensor to the target air-fuel ratio, and controls the air-fuel ratio of the internal combustion engine with the manipulated variable.
The model comprises a model expressing the object exhaust system as a discrete-time system for generating the output of the second exhaust gas sensor from the output of the first exhaust gas sensor via a response delay element and/or a dead time element, and includes, as the parameter, at least one of a coefficient relative to the output of the first exhaust gas sensor and a coefficient relative to the output of the second exhaust gas sensor.
The model mathematically expresses the output of the second exhaust gas sensor in each control cycle with the outputs of the second exhaust gas sensor and the first exhaust gas sensor in a past control cycle and coefficients of the outputs of the sensors. The output of the second exhaust gas sensor in the past control cycle corresponds to the response delay element. When the output of the first exhaust gas sensor in a control cycle prior to the dead time of the object exhaust system is used as the output of the first exhaust gas sensor in the above model, it corresponds to the dead time element.
By thus constructing the model of the object exhaust system and the values of the coefficients used in the model as the parameter are identified based on the data of the outputs of the first and second exhaust gas sensors, the identified value of the parameter (coefficients) of the model accurately reflects the actual behavioral characteristics of the catalytic converter included in the exhaust system. As a result, the reliability of the deterioration evaluating parameter which represents the degree of variation of time-series data of the identified value of the parameter is achieved as the deterioration evaluating parameter is correlated to the deteriorated state of the catalytic converter. Therefore, the deteriorated state of the catalytic converter can be evaluated adequately based on the deterioration evaluating parameter. By modeling the object exhaust system as a discrete time system, the value of the parameter (coefficients) can be identified on a real-time basis.
With the object exhaust system being thus modeled, the parameter preferably includes the coefficient relative to the output of the first exhaust gas sensor, and the step of evaluating the deteriorated state of the catalytic converter preferably comprises the step of evaluating the deteriorated state of the catalytic converter based on the deterioration evaluating parameter determined from time-series data of the identified value of the coefficient relative to the output of the first exhaust gas sensor.
Specifically, when the identified value of the coefficients relative to the output of the first exhaust gas sensor and the identified value of the coefficients relative to the output of the second exhaust gas sensor are compared with each other, the time-series data of the former identified value exhibits the above tendency against the deteriorated state of the catalytic converter more than the time-series data of the latter identified value. Therefore, the deteriorated state of the catalytic converter can be evaluated more adequately based on the deterioration evaluating parameter that is determined from the time-series data of the identified value of the coefficients relative to the output of the first exhaust gas sensor.
With the object exhaust system being thus modeled, the step of sequentially identifying the value of the parameter comprises the steps of sequentially identifying the value of the parameter according to an algorithm for sequentially updating and identifying the value of the parameter in order to minimize an error between the output of the second exhaust gas sensor in the model and an actual output of the second exhaust gas sensor, and filtering the output of the second exhaust gas sensor in the model and the actual output of the second exhaust gas sensor with the same frequency passing characteristics in calculating the error.
It is thus possible to identify the value of the parameter (coefficients) in a manner to cause the frequency characteristics of the actual object exhaust system including the catalytic converter and the model, or more specifically the frequency characteristics of changes of the output of the second exhaust gas sensor (corresponding to the output of the model) with respect to changes of the output of the first exhaust gas sensor (corresponding to the input of the model), to match each other. Thus, the identified value of the parameter is highly reliable as reflecting the behavioral characteristics of the object exhaust system including the catalytic converter. Therefore, the deteriorated state of the catalytic converter can be evaluated more adequately based on the deterioration evaluating parameter which represents the degree of variation of the time-series data of the identified value.
The step of sequentially identifying the value of the parameter preferably comprises the step of sequentially identifying the value of the parameter depending on a particular behavior of the object exhaust system.
Depending on the behavior of the object exhaust system, the identified value of the parameter may lack reliability. By identifying the value of the parameter in a certain behavior of the object exhaust system, i.e., a behavior in which air-fuel ratio of the air-fuel mixture recognized by the oxygen concentration in the exhaust gas changes from a leaner value to a richer value, the identified value of the parameter is made highly reliable as reflecting the behavioral characteristics of the object exhaust system. Thus, the reliability of the evaluation of the deteriorated state of the catalytic converter based on the deterioration evaluating parameter which represents the degree of variation of the time-series data of the identified value is increased.
The step of sequentially identifying the value of the parameter preferably comprises the step of recognizing the particular behavior of the object exhaust system based on the value of a function that is determined by a predetermined number of time-series data prior to the present of the output of the second exhaust gas sensor.
The step of sequentially identifying the value of the parameter preferably comprises the step of limiting the identified value of the parameter.
The above process makes it possible to prevent the value of the parameter from being unduly identified due to a disturbance other than the deteriorated state of the catalytic converter. As a result, the reliability of the evaluation of the deteriorated state of the catalytic converter based on the deterioration evaluating parameter is increased. As the identified value is prevented from being determined as a noisy value, the stability of the process of controlling the air-fuel ratio of the internal combustion engine using the identified value is increased.
The step of sequentially identifying the value of the parameter preferably comprises the step of calculating the identified value of the parameter based on the difference between an actual output of the first exhaust gas sensor and a predetermined reference value and the difference between an actual output of the second exhaust gas sensor and a predetermined reference value, which differences are used as the data of the outputs of the first and second exhaust gas sensors.
In calculating the identified value of the parameter, the difference between the actual output of the first exhaust gas sensor and the predetermined reference value and the difference between the actual output of the second exhaust gas sensor and the predetermined reference value are used as the data of the outputs of the first and second exhaust gas sensors. In this manner, an algorithm for calculating the identified value can be constructed relatively easily, and the accuracy of the identified value is increased.
As described above, for controlling the air-fuel ratio of the internal combustion engine in order to converge the output of the first exhaust gas sensor to a given target value, the reference value relative to the first exhaust gas sensor is preferably set to the above target value.
The step of evaluating the deteriorated state of the catalytic converter comprises the steps of determining a central value of the identified value of the parameter by effecting a low-pass filtering process on the time-series data of the identified value of the parameter, and determining the deterioration evaluating parameter from the difference between the central value and each of the time-series data of the identified value of the parameter.
Specifically, by effecting the low-pass filtering process on the time-series data of the identified value of the parameter of the model, the central value of the identified value can be determined. Inasmuch as the degree of variation of the time-series data of the identified value is closely related to the magnitude of the difference between the central value and. each of the time-series data of the identified value, the deterioration evaluating parameter which represents the degree of variation can be obtained by determining the deterioration evaluating parameter from the difference.
The low-pass filtering process preferably comprises a filtering process according to a sequential statistical algorithm.
When the central value is determined by the filtering process according to the sequential statistical algorithm, the central value of the identified value can be determined with a small memory capacity without the need for a memory for storing many time-series data of the identified value.
The sequential statistical algorithm may comprise a method of least squares, a method of weighted least squares, a degressive gain method, a fixed gain method, etc.
While the absolute value of the difference between the data of the individual identified value and the central value or the square of the difference may be used as the deterioration evaluating parameter, it is preferable to determine the deterioration evaluating parameter by effecting a low-pass filtering process on the square or absolute value of the difference between the data of the individual identified value and the central value.
The value of the deterioration evaluating parameter thus determined is highly correlated to the deteriorated state of the catalytic converter, and monotonously increases as the deterioration of the catalytic converter progresses. The variation of values of the deterioration evaluating parameter at respective deteriorated states of the catalytic converter is small, i.e., the extent of deterioration of the catalytic converter and the value of the deterioration evaluating parameter have a clear 1:1 correspondence. Therefore, the deteriorated state of the catalytic converter can be evaluated highly reliably and accurately based on the deterioration evaluating parameter.
The filtering process for determining the deterioration evaluating parameter should preferably a filtering process according to a sequential statistical algorithm as with the process of determining the central value of the identified value.
When the deterioration evaluating parameter is determined by the filtering process according to the sequential statistical algorithm, which preferably comprises a method of least squares, a method of weighted least squares, a degressive gain method, a fixed gain method, etc., the deterioration evaluating parameter as the central value of the square or absolute value of the difference can be determined with a small memory capacity without the need for a memory for storing many time-series data of the square or absolute value of the difference.
The method further comprises the step of determining whether the exhaust gas is supplied to the exhaust passage at a substantially constant rate or not, and the step of evaluating the deteriorated state of the catalytic converter comprises the step of preventing the deterioration evaluating parameter from being determined using data of the identified value if it is determined that the exhaust gas is supplied to the exhaust passage at the substantially constant rate.
Specifically if the exhaust gas is supplied to the exhaust passage at the substantially constant rate, since the output of the first or second exhaust gas sensor varies to a small extent, even when the deterioration of the catalytic converter progresses relatively largely, the variation of identified values of the parameter may possibly be relatively small. In such a situation, since the time-series data of the identified value does not properly reflect the deteriorated state of the catalytic converter, even if the deterioration evaluating parameter is determined from the time-series data, it is not suitable in evaluating the deteriorated state of the catalytic converter. According to the present invention, therefore, if it is determined that the exhaust gas is supplied to the exhaust passage at the substantially constant rate, the deterioration evaluating parameter is prevented from being determined using data of the identified value. Accordingly, the reliability of the deterioration evaluating parameter is achieved.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.